1. Technical Field
The present invention relates generally to a semiconductor memory device, and more particularly, to a nonvolatile memory device and an operating method thereof.
2. Related Art
A semiconductor memory device is typically classified as a volatile memory device or a nonvolatile memory device. The volatile memory device loses data stored therein when power supply is cut off, but the nonvolatile memory device maintains data stored therein even when a power supply is not supplied. The nonvolatile memory device may include various types of memory cells.
The nonvolatile memory device may be classified as a flash memory device, a ferroelectric RAM (FRAM) using a ferroelectric capacitor, a magnetic RAM (MRAM) using a tunneling magneto-resistive (TMR) layer, a phase change memory device using chalcogenide alloys and the like, depending on the structure of memory cells.
Among the nonvolatile memory devices, the flash memory device is roughly classified into a NOR flash memory device or a NAND flash memory device, depending on the connection state between memory cells and a bit line. Since the NOR flash memory device has a structure in which two or more memory cell transistors are connected in parallel to one bit line, it exhibits excellent random access time properties. On the other hand, the NAND flash memory device has a structure in which two or more memory cell transistors are connected in series to one bit line. Such a structure is referred to as a cell string, and one bit line contact is required per cell string. Due to the structure, the NAND flash memory device exhibits excellent properties in terms of the degree of integration.
When a program operation is performed on a memory cell of the nonvolatile memory device, the memory cell traps charges. In other words, the programmed memory cell stores charges. The longer time elapses or the read number of the memory cell increases after a memory cell is programmed, the more the memory cell deteriorates. When the memory cell deteriorates, the charges stored in the memory cell may flow out of the memory cell, even though the memory cell has nonvolatile memory properties. When the stored charges flow out of the memory cell, the reliability of data cannot be guaranteed. The characteristic of retaining stored charges within a predetermined specification is referred to as the retention characteristic of a memory cell, and since a memory cell of which the retention characteristic is degraded cannot maintain a programmed state, a read fail may occur.